Field of the Invention
The present invention relates to a process for preparing an aromatic polycarbonate resin. More particularly, the invention relates to a process for preparing an aromatic polycarbonate resin with little color by transesterification.
Aromatic polycarbonate resins have been widely used as engineering plastics in many applications for which such properties of the resins as impact resistance, dimensional stability and transparency are particularly useful. Industrial methods for the preparation of the resins include phosgenation (interfacial polymerization) and transesterification (melt polymerization).
The phosgenation method is industrially widely used wherein an alkali salt of an aromatic dihydroxy compound and phosgene are reacted in a two phase system of water and methylene chloride to produce a high molecular weight polycarbonate. In this method, however, there are problems of release into the atmosphere of methylene chloride used as a solvent in a large amount and/or residual acidic materials such as halides in the polymer produced.
The transesterification method involves the reaction of a dihydroxy diaryl compound such as bisphenol with a diaryl carbonate such as diphenyl carbonate in melt state. This method may be more preferable since it is not necessary to use highly toxic phosgene and there is no environmental pollution by halogenated solvents such as methylene chloride.
However, in the preparation of a diaryl carbonate from a dialkyl carbonate by transesterification in the presence of a phenol, the resulting diaryl carbonate may be contaminated with by-products having a boiling point close to that of the diaryl carbonate, as described in, for example, Japanese Patent Application Laid-open No. 61-172852 (1986). Further, Japanese Patent Application Laid-open No. 4-100824 (1992) discloses a method for preparing an aromatic polycarbonate by melt polymerization wherein a diaryl carbonate having a xanthone content of 10 ppm or less is employed. However, there is still a problem of coloration in aromatic polycarbonates.
On the other hand, Japanese Patent Publication No. 61-39972 (1986) discloses the use of an iminocarboxylic acid or salt thereof as an transesterification catalyst. In this method, however, the polymerization reaction must be carried out at high temperatures for a long time and, accordingly, the hue of the resulting polymer is unsatisfactory.
Further, in the conventional melt polymerization for preparing polycarbonates, since the viscosity of polymer becomes very high in later stages of reaction, the reaction temperature should be increased to produce an aromatic polycarbonate with a high molecular weight, but the hue of the polymer will worsen due to thermal deterioration.
It is known that a thermal stabilizer is added to prevent molecular weight reduction and/or discoloration when polycarbonates are melt molded.
Such thermal stabilizers include various phosphate esters as shown in Japanese Patent Publication No. 37-13775 (1962) and Japanese Patent Application Laid-open No. 58-126119 (1983), N-alkylphenothiazines as shown in Japanese Patent Application Laid-open No. 49-47459 (1974), hindered phenols as shown in Japanese Patent Application Laid-open No. 61-151236 (1986), and epoxy compounds. The use of these stabilizers could improve thermal stability of aromatic polycarbonates prepared by phosgenation to a practically useful level; in aromatic polycarbonate resins prepared by melt polymerization however, these stabilizers could not always provide an aromatic polycarbonate composition with satisfactory thermal stability due to residual metal salt catalysts in the resin.
Further, it is known that the nature of the materials for the reactor may have a significant effect on the discoloration and this tendency is stronger at higher temperature.
To provide polymers with a good hue, the type of reactor used and the nature of materials used therefor have recently been investigated. For instance, attempts have been made to produce polycarbonates by utilizing various types of high-viscosity reactors for the purpose of alleviating the heat history of polymers at later stages of the reaction and/or to select the materials for the reaction vessel to prevent the discoloration.
The reactors investigated include, for example, the engaged twin-screw extruder used in Japanese Patent Publication No. 52-36159 (1977), the paddle type self-cleaning twin-screw extruders used in Japanese Patent Application Laid-open Nos. 63-23926 (1988) and 4-106124 to 106126 (1992), and the lateral mixers used in Japanese Patent Application Laid-open Nos. 2-153923 to 153927 (1990). With the use of these reactors, the polymerization temperature could be reduced to about 270.degree. to 290.degree. C., providing an improvement in hue as compared with the case where polymerization is effected-at high temperatures to high polymerization degrees by merely using a vessel type reactor; however, resulting polymers are inevitably colored due to the metal used for the material of the proposed reactors, which is usually stainless steel.
On the other hand, attempts have been made to select a particular material used in the reactor to prevent the discoloration. For instance, U.S. Pat. No. 4,383,092 describes that a polymer with little color can be obtained by providing a cladding or lining of Ta, Ni, Cr or any mixture thereof, or glass on the internal surface of a reactor. The coloration of polymers can also be prevented by using materials containing 85% or more of copper and/or nickel as shown in Japanese Patent Application Laid-open No. 4-72327 (1992) or materials containing 20% by weight or less of iron as shown in Japanese Patent Application Laid-open No. 4-88017 (1992) to form a reactor. However, there is a problem of higher cost of plant construction in these methods since it is necessary to use an expensive alloy instead of stainless steel as a material for forming a reactor or to provide a lining on the internal surface of a reactor.
Thus, techniques for effecting the melt polymerization process at low cost have not been established even though melt polymerization is a preferable process because there is no need to use toxic phosgene or halogenated solvent with melt polymerization.